This invention relates to nuclear fuel storage and in particular to a method and apparatus for storing fuel assemblies in a pool.
Reactor fuel element assemblies are frequently stored in storage pools which can accommodate either new or spent fuel assemblies. The pool is filled with water which may be borated. This supplies cooling of the assemblies as well as moderator and also poison if the water is borated. It is of course essential that the stored mass not be permitted to assume a geometry which is either critical or supercritical.
The storage pool must be provided during the initial construction of the plant so as to provide for storage of any fuel assemblies which would have to be removed from the reactor. The storage pool at this time need not be capable of storing its ultimate capacity. Investment in expensive materials as components of the storage assembly require a present investment if supplied with the initial storage rack. There is an obvious economic saving if such investment could be deferred.
Most storage arrangements are designed for a particular fuel enrichment and are, therefore, completely inadequate should fuel of additional enrichment have to be stored at some time in the future. While borated water may be used in the pool to compensate for this additional enrichment it is considered an unsafe practice to completely rely on the boron content. In the event that the pool develops a leak and water must be replaced with fresh water, the boron content is depleted. Furthermore, there is always a potential for an operating errorwhereby the boron concentration is not maintained at the safe level.
Storage racks have been designed utilizing the flux trap principle as illustrated in U.S. Pat. No. 4,004,154 issued to Frank Bevilacqua on Jan. 18, 1977. In such a device a stainless steel plate closely surrounds the fuel assembly being stored with water contained between the plates. Fast neutrons from the fuel pass through the plates and are slowed to thermal levels by the water. At the thermal level they are not able to return through the plates to the fuel. The required spacing for a particular fuel enrichment is calculated according to well known nuclear physics principles. There is an inherent expense in holding tolerances of a structure where multiple plates are involved and tolerances must be simultaneously held.